Synthesis of perhalogenated silylboranes (X = Cl, I) and their application in regiodivergent alkene silaboration

Silaboration of olefins is a synthetically valuable and atom-economic mode of functionalization; however, it typically requires transition-metal catalysis. We have overcome this requirement by using highly reactive perhalogenated silylboranes, X₂B-SiX₃ (X = Cl, I), for which we herein report a straightforward synthesis, a full characterization, and their key properties. Access to this compound class was enabled by substantial improvement in the synthesis protocol for our previously published compound [Et₄N][I₃B-SiI₃], now available on a 40 g scale via only two steps. Cation exchange with Li[Al(OC(CF₃)₃)₄] affords the mixture Li[I₃B-SiI₃]/I₂B-SiI₃/LiI, serving as a synthetic equivalent of the elusive pure I₂B-SiI₃. Its chlorine analogue Cl₂B-SiCl₃ is accessible as a distillable liquid via treatment of [Et₄N][I₃B-SiI₃] with GaCl₃. For both perhalogenated silylboranes, various Lewis base adducts Do·X₂B-SiX₃ were obtained in excellent yields and structurally characterized by X-ray diffraction (Do = SMe₂, Py, PPh₃, IDipp; IDipp = 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene). We demonstrated that Me₂S·I₂B-SiI₃ undergoes efficient 1,2-silaboration of the challenging, non-activated substrate ethylene at rt with 0.1 eq. BI₃ as promoter. In contrast, Li[I₃B-SiI₃]/I₂B-SiI₃/LiI effects a quantitative, unprecedented 1,1-silaboration of cyclohexene at rt. This remarkable reactivity switch was elucidated by experimental and quantum-chemical studies of the underlying steric and electronic factors.